How to arrange a device to access to channel becomes increasingly important in wireless communications networks. Channel access technology is usually implemented in the Media Access Control (MAC) protocol layer, and can be divided into the following modes: frequency division multiple access, time division multiple access, code division multiple access, and random access, of which random access is a widely employed mode, particularly in ad hoc networks or mesh structured networks, for instance, the currently available wireless Local Area Network (LAN) system, Blue Tooth system and ZigBee system (a low-rate and short-distance wireless technology based on the IEEE 802.15.4 Standard), as it possesses the characteristics of simplicity, easy control and capability for being processed in a distributional way.
In accordance with the mode of random access to channel, the device monitors for a period of time prior to access to channel. If no other devices are found to be transmitting signals, the device transmits a signal or waits for a random period of time to transmit the signal. If another device is found to be transmitting a signal, the device waits for a random period of time and then monitors the channel. Specific algorithms may vary to certain extent, but it is not necessary to control at the higher layer, as each device transmits data in a rate predetermined by itself and competes for the channel in accordance with the MAC protocol entirely.
However, the mode of random access to channel may engender some problems. In the case that the transmit power of the device is given, communication capacity accommodating in a unit area is restricted. For instance, there are many devices in a unit area, and there might be different devices transmitting data in different time, but in normal cases there can be one device transmitting data over each channel at any time. Otherwise, there would be interference due to conflicts. Thus, devices having larger data traffic might occupy the channel for a prolonged period of time, thereby making it difficult for other devices to access or causing the other devices to be considerably delayed, thus making it difficult to guarantee fairness among the devices.
Moreover, the problem of “hidden node” leads to some defects in the mode of random access to channel. One characteristic of wireless communication is the fast attenuation of signals with increasing distance, so that each wireless device has a certain range of communication. As shown in FIG. 1, node B is within the range of communication of node C, whereas node A is outside the range of communication of node C. However, in the case that the network does not employ such hardware measure as a directional antenna, transmission of the signal is not directional, so that when node C transmits data to node D, node B can also receive the data. In accordance with the mode of random access to channel, it is necessary for node A or C to monitor the channel prior to data transmission, and node A or C can transmit data when no other node is transmitting data so as to avoid conflict. But in the circumstance as shown in FIG. 1, since node A is not in the same range of communication as node C, upon monitoring the channel, node A cannot “hear” that node C is transmitting data, and hence node A transmits data to node B, at which time node B cannot properly receive the data due to interference from node C. In other words, signals sent from node A and node C conflict with each other at node B, and node C is called in this case as a “hidden node” of node A. Similarly, node A may also become a hidden node of node C.
Existence of the “hidden node” not only leads to influence by other irrelevant devices to the current device, but also causes interference among several devices on the same communication link. As shown in FIG. 2, the communication link is A->B->C and employs a single communication channel. Data transmission cannot be carried out in the maximum rate of 100% on this link, because when device A transmits data to device B, device B cannot transmit data to device C. Otherwise the transmitter signal of device B would greatly interfere with its own receiver. As a result, the maximum rate is only 50%, that is to say, device A sends data to device B in 50% of the time, and device B sends data to device C in the remaining 50% of the time.
In the case of three-hops, the adverse influence of “hidden node” on the communication rate is more salient than in the case of two-hops as shown in FIG. 2. As shown in FIG. 3, the communication link is A->B->C->D and employs a single communication channel. Under such a circumstance, the rate is at most one third of the maximum rate. When device A transmits data to device B, the transmitter signal of device B interferes with its data reception, and device can hence not transmit the signal, while device C, as a “hidden node” of device A, cannot transmit data to device D either. When device B transmits data to device C, both devices A and C cannot transmit data, this is because transmitter signals of devices B and C interfere with their data reception. That is to say, the data transmitted from device A to device B is interfered, while the signal transmitted from device C to device D interferes with device C itself. Similarly, when device C transmits data to device D, both devices A and B cannot transmit data. Consequently, the entire link is forced to divide into three segments, each of which makes use of at most one third of the time to communicate, so that the rate of the entire link is restricted to be one third of the maximum rate. In cases of more hops, if no hardware measure such as a directional antenna is used, the rate will be restricted to be one third of the maximum rate even under the best conditions.
As mentioned above, existence of the “hidden node” leads to interference among devices and restricts communication rates of the devices. With respect to a device employing random access to channel, communication rate cannot be guaranteed because the occurrence of such interference is unpredictable, so that it is impossible to guarantee service delay to meet the requirements.